A hydraulically induced fracture containing bitumen was encountered in the Colorado Shale at Imperial Oil's Cold Lake Operation, during development drilling in 1997. The fracture was apparently caused by an inadvertent release of fluids from Cyclic Steam Stimulation (CSS) operations in the Clearwater formation into the shale about 150 m above the producing formation. Subsequent drilling delineated the fracture to be over 1 km in diameter, extending over five 20-well pads. Steam injection into the Clearwater formation induces overburden heave and also induces additional shear stresses in the shale. These could cause the shale to slip along the fracture. Depending on the magnitude of the slip, casing strings could be deformed or even failed. Numerical models were developed to assess the risk of future CSS operations and to optimize steaming strategies at the affected pads. A coupled geomechanical-thermal- reservoir simulation code, GEOSIM, was linked to the thermo-elasto-plastic capabilities in the fine element code, ABAQUS, to model the reservoir and overburden, including contact or slip elements in the fracture layer. Field measurements of the fracture pressure and laboratory measurements of the shear strength of the shale were important inputs to the model. Subsequent CSS cycles were conducted with pressure and temperature monitoring of the shale at the fracture depth and microseismic monitoring of the entire shale. Poroelastic fluid pressure and passive seismic responses in the fracture were observed during steaming and were consistent with the numerical modelling. Successful completion of three high pressure CSS cycles at pads with moderate shale fracture pressure allowed for steaming of a pad with higher shale fracture pressure. This case study is an excellent example of integrating technical geomechanics modelling with operations optimization. Introduction Imperial Oil Resources has been conducting commercial operations at the Cold Lake site since 1985. The site is located approximately 300 km NE of Edmonton, Alberta. The Cyclic Steam Stimulation (CSS) process is used to produce bitumen from the Clearwater (CW) formation which typically is found at depths between 420 m and 470 m in this area(1). More than 3,000 deviated wells, typically arranged in pads of 20 wells, are used to inject high pressure steam (>10 MPa) to reduce the viscosity of the bitumen. The same wells are then used to produce a mixture of bitumen, water, and gas. A hydraulically induced fracture containing bitumen was encountered in the Colorado Shale (CS), during development drilling of the E07 pad in 1997. Fifteen Shale Evaluation Wells (SEW) were drilled through the CS to determine the extent of the fracture. Evidence of the fracture was found in wells drilled from five neighboring pads of CSS wells. The evidence consisted of abnormally high fluid pressures, bitumen in the drilling returns, or, in the case of the initial observation, flow of bitumen to surface. An interpolated sketch of the fracture based on the results of the drilling is shown in Figure 1. It was concluded that this was a single contiguous fracture based on the consistency of observed depths and the magnitude of the observed pressures.
Summary This paper describes differences between actual material behavior and idealizations used for modeling purposes and discusses some of the implications for interpreting model predictions. Much of the design for well structures subjected to high-amplitude cyclic loading is based on material assumptions that extrapolate strength properties from uniaxial, tensile tests to conditions where multiaxial, cyclic stresses are imposed. This paper presents results from cyclic testing on a common oil-country-tubular-goods (OCTG) material and demonstrates differences between the physical behavior measured under cyclic loading conditions and theoretical behavior extrapolated by numerical modeling. Modeling theories for plastic deformation are discussed with their limitations and relevance in a cyclic-loading environment. The implications of these limitations for design choices in thermal wells also are discussed with example applications of cyclic material behavior and fatigue-life prediction. Material fatigue properties for the high-amplitude, low-cycle application of thermal operations have not been investigated in much depth previously, particularly for OCTG. Along with characterizing cyclic mechanical properties, the tests discussed here also assessed the low-cycle fatigue properties of the sample OCTG steel. The consistent fatigue measurements, combined with analysis results using representative cyclic mechanical properties, can provide a basis for estimating fatigue life. Depending on analysis-model assumptions, substantial variation in predicted fatigue life can occur; therefore, exact fatigue-life predictions are not anticipated. The primary value in such modeling is in evaluating the relative effectiveness of mitigation options for extending well life. Introduction Most thermal enhanced-oil-recovery (EOR) wells in western Canada operate using either the cyclic-steam-stimulation (CSS) or the steam-assisted-gravity-drainage (SAGD) method. In both methods, operational factors result in thermal cycles being imposed on the well structures, particularly in the intermediate casing (Placido et al. 1997). Thermal expansion is constrained by the formation and cement in CSS and SAGD wells, producing loads that exceed the yield strength of the tubulars when the well is heated. Localization mechanisms also might amplify the strain magnitude, imposing additional plastic fatigue load at discrete locations along the well structure. Thermal-well casing designs have evolved during more than 30 years of operating experience, and much of the computer modeling that describes casing performance is based on measured uniaxial tensile material properties that are extrapolated to multidimensional cyclic behavior through engineering models. Cyclic material-properties data are sparse, particularly in the temperature regime common in thermal-recovery wells. Furthermore, plastic fatigue-life information for materials commonly used in well construction is difficult to obtain. Such information, however, is required to make reliable predictions of certain deformation mechanisms and the associated fatigue life for wells exposed to cyclic, thermally imposed loading. A test program for characterizing cyclic material properties was implemented to evaluate both cyclic mechanical properties and low-cycle fatigue life. Test-result consistency indicates a reliable material characterization that can be applied in constitutive analysis models and component-life assessments. The observed cyclic-stress-strain material behavior also demonstrates different characteristics from those predicted through engineering models using uniaxial monotonic material properties for input. This has important implications for thermal-well design and operations.
fax 01-972-952-9435. AbstractMuch of the design basis for well structures subjected to high amplitude cyclic loading is based on material assumptions that extrapolate strength properties from uniaxial, monotonic tests to conditions where cyclic, multiaxial stresses are imposed. This paper shows results from cyclic testing on common Oil Country Tubular Goods (OCTG) materials and demonstrates the difference between physical behavior measured under cyclic loading conditions and theoretical behavior extrapolated by numerical modeling of uniaxial, unidirectional test data. Modeling theories for plastic deformation are discussed, along with their limitations and relevance in a cyclic loading environment. The implications of these limitations for design choices in thermal wells also are discussed.Fatigue properties for the high-amplitude, low cycle application of thermal operations have not previously been investigated in much depth, in particular for OCTG. Along with characterizing cyclic mechanical properties, the tests discussed here also were used to assess the low-cycle fatigue properties of steel commonly used for in thermal well casings. Consistent fatigue results were produced, which, applied in the context of analysis results using representative cyclic mechanical properties, provide a basis for estimating fatigue life for specific cyclic deformations. Depending on scenario assumptions, substantial statistical variation in fatigue life can be expected, so exact fatigue life predictions are not anticipated. The primary value in such modeling capability is the assessment of mitigation options for extending well life when casing deformations are indicated.The paper also discusses some practical implications of the difference between actual material behavior and idealizations used for modeling purposes. An example application of the cyclic material behavior and fatigue life prediction is also included.
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